1. Field of the Invention
The invention relates to a fuel injection system for an internal combustion engine that has at least one cylinder.
2. Description of the Prior Art
A fuel injection system for multicylinder internal combustion engines in which an injection unit is associated with each cylinder and is supplied with fuel at a relatively high pressure from a high-pressure fuel supply is disclosed in DE 44 14 242 A1. In this prior art system, a through flow limiting valve is disposed upstream of the injection unit in the high-pressure fuel supply and closes when the fuel flowing out of the through flow limiting valve reaches a maximal fuel quantity. This is intended to prevent damage to the engine due to a leak in the injection unit.
DE 197 09 794 A1 describes an injection unit in detail, in which an injection valve is supported in axially mobile fashion, which has a valve needle and a piston that is drive-connected to this valve needle, where the valve needle is disposed between an injection nozzle, which feeds into the combustion chamber of the cylinder, and a nozzle supply line, which communicates with the high-pressure fuel supply, and where, at an end remote from the valve needle, the piston protrudes with a closing surface area into a closing pressure chamber and at an end oriented toward the valve needle, protrudes with an opening surface area into an opening pressure chamber. While the closing pressure chamber communicates with the high-pressure fuel supply via a throttle, the opening pressure chamber is connected so that it communicates in an unthrottled manner with the high-pressure fuel supply. Furthermore, a control valve is disposed between the closing pressure chamber and a relief chamber in which a relatively low pressure prevails. The closing surface area is selected to be greater than the opening surface area. When the control valve is closed, the same (high) pressure prevails in the opening pressure chamber as in the closing pressure chamber so that due to the above-mentioned area difference, a resulting force is exerted on the piston, which drives the injection valve into its closed position. As soon as the control valve is opened, for example by means of an electrically actuated Piezoelectric actuator, the closing pressure chamber and the relief chamber are connected to each other in a communicating fashion so that a pressure drop occurs in the closing pressure chamber. Since the pressure in the opening pressure chamber remains at the same level, a resulting force acts on the piston and drives the injection valve into its open position. In order to open the injection valve, the valve needle is pulled out of a valve seat embodied in the injection nozzle so that the fuel can be injected into the combustion chamber at a high pressure.
Since in the known fuel injection systems, the through flow limiting valve is disposed in the high-pressure fuel supply upstream of the injection unit, the maximal fuel quantity defined in the through flow limiting valve must be selected so that it includes the following partial fuel quantities: the maximal injection quantity that is required for the engine in order to be able to produce its maximal power in all operating states. Another partial fuel quantity arises from the control quantity that is required in order to be able to carry out the pressure relief of the closing pressure chamber, in which fuel flows into the relief chamber, for the purpose of opening the injection valve. Finally, the still tolerable leakage quantity for the respective injection unit, which is in particular composed of leaks in the injection valve and in the control valve, must be taken into account as an additional partial fuel quantity. This relatively high maximal fuel quantity must be reached so that a malfunction of the injection unit, for example a valve that is blocked in its open position, or a no longer tolerable leakage, results in the fact that the through flow limiting valve closes the fuel supply to the injection unit. Smaller leaks can consequently lead to a reaction of the through flow limiting valve only in those operating ranges of the engine in which a relatively large injection quantity is required.
The fuel injection system according to the invention, has the advantage over the prior art that the maximal fuel quantity can be reduced so that the through flow limiting valve reacts earlier, which better protects the engine from damage.
In the fuel injection system according to the invention, the maximal fuel quantity can be reduced since the through flow limiting valve is disposed in the nozzle supply line and consequently operates independent of leaks in the control valve. Correspondingly, the maximal fuel quantity in the fuel injection system according to the invention can be reduced by the relatively large control quantity and by the leakage quantity that is tolerable for the control valve. Accordingly, the new maximal fuel quantity is only slightly greater, namely by the leakage quantity that is tolerable for the injection valve, than the maximally required injection quantity.